Black Hills by Nora Roberts

Black Hills - Cooper Sullivan is angry. His parents are off to Europe on a trip trying to save their marriage and he is being sent to South Dakota to stay with his grandparents for the summer. While there, he meets Lil Chance. Lil makes the visit not so bad. On this 1st visit Lil spots a cougar. Coop returns for summer visits throughout his teen years and Lil and his friendship grow. On his last summer before college, they encounter a tragedy.

When Cooper returns several years later to help his grandparents, Lil has achieved her hopes, a wildlife refuge. However, there is a killer nearby that threatens not only Lil, but the refuge as well.